1. Field of the Invention
The present invention relates to power switching devices, and particularly to an improvement for compatibly suppressing the occurrence of switching loss and the occurrence of surge voltage when connected to an inductive load.
2. Description of the Background Art
&lt;Structure of Conventional Device&gt;
FIG. 6 is a circuit diagram showing a conventional power switching device using an insulated gate bipolar transistor element (an IGBT element) and peripheral devices thereof. The IGBT element 6 included in the power switching device turns on and off between two main electrodes, that is, a collector electrode C and an emitter electrode E to turn on and off a load circuit connected to these main electrodes. An inductive load 8 and a DC power source 10 are connected to the load circuit. The inductive load 8 and the DC power source 10 are provided outside the power switching device.
A freewheeling diode 7 is connected to the inductive load 8 in parallel and a snubber capacitor 9 is connected to the DC power source 10 in parallel. The freewheeling diode 7 and the snubber capacitor 9 form a surge absorption circuit for absorbing the voltage surge due to the inductive load 8. Inductances 11 and 12 parasitically occur in the wiring connecting the DC power source 10, the freewheeling diode 7 and the snubber capacitor 9.
Emitter electrodes E of two transistors 3a and 3b are connected to the gate electrode G of the IGBT element 6 through a resistance 4. The collector electrodes C of these transistors which are mutually complementary are connected to a DC power source 5. The low potential side output of the DC power source 5 is connected to the emitter electrode E of the IGBT element 6. A base resistance 2 is connected to the base electrodes B of the transistors 3a and 3b. A pulse generator 1 is connected between one end of the base resistance 2 and the emitter electrode E of the IGBT element 6. The pulse generator 1 is provided outside the power switching device.
The transistors 3a, 3b, the resistances 2, 4 and the DC power source 5 form a control circuit for turning on and off the IGBT element in response to pulse signals inputted to the base electrodes B through the base resistance 2.
&lt;Operation of Conventional Device&gt;
The conventional device, which has such structure as described above, operates as described below. When the pulse generator 1 inputs a high voltage signal (15 V, for example) to the base resistance 2, the transistor 3a turns on and the transistor 3b turns off. The current (ON driving current) supplied from the DC power source 5 is then provided to the gate electrode G through the transistor 3a, and the voltage between the gate electrode G and the emitter electrode E, i.e., the gate voltage exceeds a gate threshold voltage peculiar to the IGBT element 6, and thus the IGBT element 6 turns on. The IGBT element 6 turns on to turn on the load circuit, and then the load current is supplied from the DC power source 10 to the inductive load 8.
When the pulse generator 1 inputs a low voltage signal (0 V, for example) to the base resistance 2, the transistor 3b turns on and the transistor 3a turns off. Then the current (OFF driving current) in the direction opposite to the ON driving current is supplied to the gate electrode G, and the voltage between the gate electrode G and the emitter electrode E, i.e., the gate voltage becomes lower than the gate threshold voltage, and thus the IGBT element 6 turns off. The IGBT element 6 turns off to turn off the load circuit, and then the supply of the load current from the DC power source 10 to the inductive load 8 stops.
As the inductive load 8 has a function of holding the current corresponding to the magnitude of the inductance, the load current does not immediately become 0 when the IGBT element 6 turns off from ON, but attenuates while flowing back in the surge absorption circuit formed of the freewheeling diode 7 and the like. The surge absorption circuit is provided to prevent application of excessive surge voltage between the collector electrode C and the emitter electrode E of the IGBT element 6 when the IGBT element 6 turns off from on by providing the inductive load 8 with a path for flow-back of the load current.
The conventional art, however, has such a problem as described below. That is to say, the parasitic inductance 11 is caused on the surge absorption circuit as described above. When the IGBT element 6 turns off from on, the current flowing through the parasitic inductances 11 and 12 rapidly increases as the load current flowing though the inductive load 8 starts flowing back to the surge absorption circuit. The rapid increase in the flow-back current causes spike-like high voltage in the parasitic inductances 11 and 12.
As a result, when the IGBT element 6 turns off from on, the surge voltage is applied between the collector electrode C and the emitter electrode E of the IGBT element 6. That is to say, the conventional power switching device has a problem that the original object of the surge absorption circuit is not achieved enough because of the existence of the parasitic inductances 11 and 12.
It is an effective way for reducing the surge voltage to make the transition of the IGBT element 6 from ON to OFF, i.e., to slow down the switching speed. If the switching speed is slowed down, however, another problem arises. That is, the power loss in the switching operation in the IGBT element 6, i.e., the switching loss, increases. The conventional power switching device has a problem that the two requirements, i.e., the decrease in the surge voltage and the decrease in the switching loss, contradict each other and it is difficult to simultaneously satisfy the both.